There is a growing consensus that the collection of solar energy and its conversion to electrical energy by means of photovoltaic devices should be included in the energy mix of the near future. The commercialization of photovoltaic devices depends on technological advances that lead to higher efficiencies, lower cost, and stability of such devices. The cost of electricity can be significantly reduced by using solar modules constructed from inexpensive thin film polycrystalline semiconductors such as copper indium diselinide or cadmium telluride. Both materials have shown great promise, but certain difficulties have to be overcome before their commercialization.
Based on its physical properties, cadmium telluride has been recognized for some time as one of the leading candidates for photovoltaic applications. However, the difficulty of achieving a low-resistance ohmic contact to cadmium telluride solar cells has hindered the commercialization of these devices. An economical solution to this problem would clear one of the remaining hurdles of commercializing cadmium telluride photovoltaic modules.
The difficulty in obtaining a low resistance ohmic contact with thin films of p-type polycrystalline Class IIB-VIA semiconductors is partially due to the fact that such films exhibit relatively high resistivities. Bypassing this problem is usually achieved by lowering the semiconductor surface resistivity. Typically, acidic solutions such as HNO.sub.3 +K.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 O, HNO.sub.3 +H.sub.3 PO.sub.4 +H.sub.2 O, H.sub.2 SO.sub.4 +K.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 O and bromine methanol are used to etch the semiconductor. These solutions have been found to leave a surface rich in Class VIA element. Such a surface results in an enhanced electrical contact between a metal and the p-type semiconductor.
Nevertheless, contacts formed to the thin films of cadmium telluride using metals are in general unstable due to metal diffusion into the polycrystalline semiconductor. While a portion of the metal reacts with the semiconductor surface to form an electrical contact, the excess metal diffuses into the semiconductor creating shorts and resulting in degradation of the properties of the photovoltaic device. Limiting the amount of metal deposited, so that diffusion into the semiconductor will have a negligible effect has been found to produce satisfactory results. However, because the thickness of the metal has been reduced, the resulting metal film has a relatively high resistivity (sheet resistance).
As disclosed in U.S. Pat. No. 4,680,611, a second metal layer that forms a stable contact with the first metal is needed in order to further reduce the overall contact sheet resistance and improve solar cell characteristics. A first layer of copper and a second layer of nickel is disclosed as the preferred metal combination for a contact to a IIB-VIA thin film semiconductor.
U.S. Pat. No. 4,735,662 discloses, however, that copper-nickel contacts to CdTe film are unsatisfactory. This patent discloses that because nickel has a relatively low resistivity, it must be deposited in a relatively thick layer to achieve acceptable resistance. Thick films of nickel are mechanically unstable and will oxidize over time and flake off under the influence of internal stresses. U.S. Pat. No. 4,735,662 discloses that satisfactory copper-nickel contacts can be produced by depositing a relatively thin layer of nickel, followed by an isolation layer to protect the nickel layer from oxidation and to provide sufficient metal thickness for bonding of an external contact. The isolation layer may be formed of carbon.
Rather than contacting the semiconductor layer directly with a metal, another method of forming an ohmic contact to the IIB-VIA semiconductor is to deposit a second IIB-VIA semiconductor onto the first semiconductor layer. This second semiconductor layer presents no obstacles to the formation of a low resistivity contact. A requirement for selecting this second semiconductor is that the interface of the two semiconductors does not impede current flow. Zinc telluride and mercury telluride are examples of such semiconductors used for forming low resistance ohmic contacts with cadmium telluride.
Graphite paste has been used as a contact to CdTe solar cells, but this contact suffers from a high series resistance. To improve the resistance of the contact, salts of silver, copper, gold and mercury have been added to the graphite paste. The results, however, have been less than satisfactory because of poor diode characteristics.
Mercury telluride has been used as an ohmic contact to thin film cadmium telluride solar cells. P-type HgTe is deposited by evaporation onto the surface of p-type CdTe. The resulting HgTe layer provides a low resistance contact to the CdTe, but this process is expensive and difficult to control.
It is therefore an object of the present invention to provide a low resistance, economically feasible ohmic contact to thin film p-type semiconductor Class IIB-VIA compounds.
It is a further object of the present invention to provide a method for forming such contacts.
These and other objects of the present invention will become apparent to those skilled in the art in the following description of the invention and in the appended claims.